The proposed work is intended to study the mechanism of action Vitamin D3 metabolites, on bone, in vitro. This would be accomplished by testing the effect of Vitamin D3 metabolites on cyclic-AMP and calcium uptake of isolated and cultured bone cells. Another objective is the development of a bioassay method for 1,25 - (OH)2-D3 in plasma, using 45Ca prelabeled rat fetal bones, in vitro. Also, study of the interaction of the different Vitamin D3 metabolites and derivatives on the isolated rat fetal bone.